Zeolites are microporous materials formed by TO4 tetrahedra (T=Si, Al, B, P, Ti, Ge . . . ) interconnected by oxygen atoms, creating pores and cavities with a uniform size and shape in the molecular range (3-15 Å). These zeolite materials have important applications as catalysts, adsorbents or ion exchangers.
These materials are used as catalysts in numerous chemical processes, wherein the use of a determined zeolite with specific physico-chemical properties for a determined chemical process will directly depend on the nature of the reactants and products involved in the process (such as size, shape, hydrophobicity . . . ) and also on the reaction conditions. In addition, the nature of the reactants and products will affect the diffusion of these molecules in the pores and cavities of the zeolites, and consequently, the selection of the zeolite with a pore topology suitable for the products involved in the reaction is essential. In addition, the zeolite must be chemically and structurally stable under the required reaction conditions.
The nitrogen oxides (NOx), mainly emitted by automobiles during the consumption of fossil fuels, has become a serious environmental problem since they are one of the biggest contaminants of the air. In this context, selective catalytic reduction (SCR) of NOx with ammonia has been demonstrated as an efficient form of control of said undesired emissions.
Amongst the more than 200 zeolitic structures accepted by the International containing copper atoms in extra-network positions, have excellent catalytic activity for selective catalytic reduction (SCR) of NOx, in addition to high hydrothermal stability (Bull, et al., U.S. Pat. No. 7,601,662, 2009; Moliner, et al., WO2013/159825, 2013). Amongst all the known zeolites with small pores, chabazite (code CHA assigned by the IZA) has received special attention. This material is formed by a tridirectional system of small pores (8A) interconnected by large cavities and also has double rings of 6 members (DA6).
The CHA zeolite in the silicoaluminate form thereof containing Cu atoms is an excellent catalyst for selective catalytic reduction (SCR) of NOx using ammonia as a reducing agent in the presence of oxygen. The incorporation of types of Cu is conventionally carried out by means of post-synthetic ion exchange treatments of the previously synthesized and calcined zeolite. This method require various steps, such as hydrothermal synthesis of silicoaluminate, calcination of the material to eliminate the OSDA, transformation to ammonium form, ion exchange of metal, and lastly, calcination in order to obtain the final zeolite with the desired metal. All these steps help to increase the total cost in obtaining the catalytic material.
In recent years, direct synthesis of silicoaluminate with a CHA structure containing copper atoms has been described using, as the only OSDA, an organometallic complex formed by copper and a polyamine with the aim of introducing cationic species of copper into the cavities of the CHA after calcining the sample in air (Chem. Commun, 2011, 47, 9783; Chin. J. Catal. 2012, 33, 92). However, the molar ratios of Si/Al obtained in the final solids are low (Si/Al˜4-7). These low Si/Al ratios can pose problems in deactivating the catalyst by irreversible dealumination processes in the conditions required for SCR of NOx (vapor presence and high temperatures). In addition, these Si/Al ratios in final solids are smaller than those introduced into the synthesis gels, in particular the difference is greater when high Si/Al ratios are studied (for example in order to obtain a Si/Al ratio in the final solid of 7.6, a theoretic ratio in the gel of 17 is required, see example Cu-ZJM4-35 in Table 1 of the publication Chem. Commum. 2011, 47, 9783). These differences between the Si/Al ratios indicate that part of the Si species introduced into the synthesis medium remain in solution and are not capable of being incorporated into the final solids, resulting in low solid yields (less than 50% when Si/Al ratios greater than 7 are used). In addition, by means of this synthesis method, it is also not possible to control the quantity of copper incorporated into the zeolite, always obtaining values greater than 9% by weight. In general, it is widely acknowledged in the literature that the greater the content of copper in the zeolitic samples, the lower the hydrothermal stability the catalyst presents (Chem. Commun., 2012, 48, 8264). Definitively, this synthesis methodology does not allow the Si/Al ratio and the Cu content in the catalysts to be controlled, properties which are very important for the activity and stability of the same.
The preferred OSDA for the synthesis of CHA zeolite in the silicoaluminate form thereof is the cation N,N,N-trimethyl-1-adamantammonium (TMAdA) (Zones, U.S. Pat. No. 4,544,538, 1985, assigned to Chevron). Recently, the use of benzyltrimethylammonium (BzTMA) as an efficient OSDA for the synthesis of the silicoaluminate form of CHA has also been described (Miller et al., U.S. Pat. No. 8,007,764, 2011, assigned to Chevron).
Taking into account the capacity of the TMAdA and BzTMA cations to direct the formation of CHA, the direct synthesis of the CHA structure in the silicoaluminate form thereof containing Cu atoms has recently been described, using specific mixtures of said OSDAs with a copper complex (Trukhan et al., U.S. Patent 2011/0076229, 2011; and Moliner et al., WO2014/090698, 2014). These descriptions allow the synthesis of these materials to be directed in greater Si/Al ranges, however they have certain disadvantages. The methodology described by Trukhan et al. always requires the combined use of two organic molecules, selected from TMAdA, BzTMA and tetramethylammonium, in addition to the organometallic copper complex formed by the addition of ammonia with a copper salt (Trukhan et al., U.S. Patent 2011/0076229). With the aim of avoiding the combined use of various OSDAs and in addition to avoiding the use of ammonia in the preparation of the material, which is extremely caustic and dangerous, in Moliner et al., the direct synthesis of the CHA structure in the silicoaluminate form thereof with copper atoms is described using one single organic molecule (TMAdA) together with a copper complex formed by a commercial linear polyamine (tetraethylenepentamine, TEPA) and a copper salt. However, the TMAdA cation has a high cost, making the process for obtaining the desired material notably expensive and limiting the possible commercial applications of this zeolite.
Therefore, in spite of the advances shown in the direct synthesis of the CHA material in the silicoaluminate form thereof containing copper atoms, there is still a need for the industry to reduce the preparation costs of this material by using other, more economic OSDAs for the preparation thereof.